Some very long-used ones in my part of the country are -
door's open but nobody home
a few sandwiches short of a picnic
a few bricks short of a full load
a bit light in the loafers
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And in regard to the recent replacement of a level name, myself I did prefer McPosterpants because to me it's funnier and I didn't find it offensive in any way. However, since that did bother another member, so be it.
Yes, I've seen a 'put on your big-girl panties' type of phrase any number of times in various settings and never have considered that offensive. It's a remark about handling a task or issue in a mature way, not a gender slur. AFAIR I've always seen it addressed from one woman to another, never from a man to a woman. That's probably also why guys may have seen 'big-boy pants' used instead.
Unless something's frankly blatant and explicit, any personal affront or distaste is a very individual reaction. Thus, my reaction only counts for me alone. Others' reactions may well be quite different on any number of topics than are mine.
Is there any other suggestion which might be more amusing than McPostington, for switching again?
(loved Don's post about McPostington, BTW.)